1. Field of the Invention
The present invention relates to insulation or protection coverings for electric components, such as terminations for electric cables, voltage dischargers or composite insulators. In particular, the present invention concerns coverings adapted to be fitted and elastically clamped to an electric component. It also pertains to the related manufacturing method. More specifically, this invention relates to elastic terminations for cables comprising said coverings and the method of mounting them.
2. Description of the Related Art
In an embodiment known from xe2x80x9cNew Prefabricated Accessories for 64-154 kV Crosslinked Polyethylene Cablesxe2x80x9d (Underground Transmission and Distribution Conference, 1974, pages 224-232), an outdoor termination comprises a base plate to which the base of an insulating covering formed of a finned body made of porcelain is linked. A cable conductor is connected to the upper end of the outdoor termination through appropriate support and connection means. A ground electrode and a field control cone of polymeric material is forcedly fitted onto the surface of the cable insulator within a cylinder at its entry into the finned body. The cylinder is made of an epoxy resin. The free space within the finned body is filled with an insulating oil.
However, filling with insulating oil may cause some problems. This insulating oil within the porcelain covering aims at eliminating air. The air may be subjected to an ionization phenomenon causing the electric field to be higher. Such ionization will impair the termination integrity.
CIGRE"" 1992, 21-201, entitled xe2x80x9cSynthetic Terminations for High Voltage Cables-Assessment of Service Lifexe2x80x9d teaches a method of replacing the porcelain material in the termination by tubes made of fibre-glass-reinforced plastics coated with anti-tracking insulating rubber. The tubes may shape like a cylinder and/or a cone. The tubes must both ensure protection for the underlying portion against humidity penetration and provide the necessary leak line (profile and fins) for surface currents. In this case, however, insulating oil is still used.
For the purposes of the present application, xe2x80x9ctrackingxe2x80x9d, or surface track, means an irreversible degradation of the insulating material surface, involving formation of conductive paths even under dry conditions. This definition is consistent with the specification of IEC 1109 of 1992 entitled xe2x80x9cComposite insulators for a.c. overhead lines with a nominal voltage greater than 100 OV. Definitions, test methods and acceptance criteria.xe2x80x9d
Examples of coatings made of anti-tracking insulating rubber for electric components are known in the art. These coatings for extruded-insulator cables are called xe2x80x9cdry terminationsxe2x80x9d because they are devoid of both the porcelain covering and the insulating oil. These coatings are described in an article entitled xe2x80x9cOutdoor and incorporated terminations for extruded synthetic cables up to 400 kVxe2x80x9d by F. Gahungu, J. M. Delcoustal, J. Brouet, presented to Jicable 1995. This article describes applications for internal use in cabins (for voltages up to 90 kV), as well as applications for temporary outdoor connections.
European Patent Application No. EP 95 106 910.3 describes a self-bearing dry termination for an electric cable intended for outdoor use for voltages of 10 to 245 kV and above. The electric cable is provided with a covering of elastomeric material fitted onto a tubular element. However, when high-voltage applications are concerned, involving voltages as high as 60 kV and above, this covering can have an early decay if its fitting takes place under strong interference conditions.
Among the materials used to make insulating coverings for applications to outdoor terminations, two fundamental types can be identified: elastic materials and heat-shrinkable materials. The elastic material consists of elastomers having the capability to make manufactured articles adapted to be interference-fitted onto the cable end or a similar support body and to keep a preestablished pressure against it. On the contrary, the heat shrinkable materials enable manufactured articles to be pre-expanded at the factory by using heat sources, generally in the form of a gas flame.
Examples of known devices adopting heat-shrinkable materials include those described in an article entitled xe2x80x9cHeat-shrinkable terminations for 66 kV polymeric cablesxe2x80x9d by J. W. Weatherley R. A. John, M. H. Parry, presented to the IEE London Power and Accessories 10 kV-180 kV, November 1986.
Although heat-shrinkable products are still in use, they are not quite desirable. Employment of a flame as required by heat-shrinkable products involves problems not only for practical-use and safety reasons, but also because of the difficulty in ensuring a correct elastic recovery of the covering to the starting sizes, especially when heating is not carried out with particular care and skill.
xe2x80x9cElasticxe2x80x9d or xe2x80x9ccold-shrinkablexe2x80x9d products preferably comprise EPR-based blends or silicone rubber-based blends.
For the purposes of the present description, EPR means blends based on ethylene-propylene polymers in particular comprising EPM (ethylene-propylene copolymer) or EPDM (ethylene-propylene-diene terpolymer) based blends.
European Patent Application No. 92203797.3 describes a tubular element which is formed by a wound strip having adjacent coils made by cutting the surface of the element itself.
European Patent Application No. 90119273.2 describes a storable element of a device to make cable joints. The cable joints consist of a tubular support over which a covering for conductor connections is fitted under elastic expansion conditions. The covering is suitable to make joints between cables selected in a wide range of diameters.
This covering comprises two coaxial and superposed sleeves. The radially inner one has a residual deformation under imposed elongation conditions. The residual deformation of the inner sleeve is lower than that of the radially outer sleeve. The inner sleeve also has a greater elastic modulus.
However, this covering is not adapted to construct terminations and the like which are exposed to phenomena such as surface discharges (tracking).
The invention comprises a tubular elastic covering for electric components. The elastic covering has predetermined values of mechanical and electric/environmental requirements. The mechanical requirements comprise expandibility and elastic recovery in a radial direction. The electric/environmental requirements comprise resistance to tracking and resistance to solar radiation. The covering comprises an inner insulating layer and an outer insulating layer. The inner and outer insulating layers are formed coaxially with each other, superposed and bonded together. The inner and outer insulating layers are made of blends of cross-linked polymeric materials. The cross-linked blend forming the inner layer has predetermined values of the mechanical requirements. The cross-linked blend forming the outer layer has predetermined values of the electric/environmental requirements.
This application has solved the problems by replacing porcelain with polymeric and composite materials in particular in medium- and high-voltage outdoor terminations of extruded cables. Several reasons for using the polymeric and composite materials include:
reducing to the minimum explosion risks in case of fire or internal electric discharge;
reducing weight;
reducing brittleness, so as to prevent damages caused by accidental impacts or, for example, vandalic acts;
increasing transportation and installation simplicity and facility;
increasing safety margins under high-pollution conditions.
This application has considered coverings of the elastic type to be applied to the surface of a cable insulator. However, the requisite qualities to be simultaneously met by the elastic coverings of the above described type are hardly compatible with each other. In particular, it has been noted that an insulating covering for cable terminations of the elastic type must meet requirements of the mechanical type, including expandibility and elastic recovery, as well as requirements of the electric/environmental type, including resistance to tracking and solar radiation.
Therefore, one aspect of the present invention relates to the combination of two different blends: one insulating blend for the inner layer (not directly in contact with the weathering agents), characterized by suitable mechanical properties including expandibility and elastic recovery; and a second blend for the outer layer suitable to withstand environmental factors affected by pollution including resistance to tracking and to weathering agents including solar radiation. The inner and outer layer are constructed under an expansion state. Resistance to tracking is evaluated on flat test pieces by the method described in specification IEC-587, second edition, 1984. No particular properties of elastic recovery is however required.
Therefore, in accordance with a first aspect, the present invention relates to a tubular elastic covering for electric components, on the whole having predetermined values of mechanical and electric/environmental requirements, in which mechanical requirements comprise expandibility and elastic recovery in a radial direction, and electric/environmental requirements comprise resistance to tracking and resistance to solar radiation, characterized in that said covering comprises an inner insulating layer and an outer insulating layer, coaxially with each other, superposed and bonded together, made of cross-linked polymeric material blends, in which the cross-linked blend forming the inner layer has predetermined values of said mechanical requirements and the cross-linked blend forming the outer layer has predetermined values of said electric/environmental requirements.
In particular, because the outer layer is arranged over a greater diameter than the inner layer, the outer layer undergoes mechanical stresses in a more limited manner. The outer layer submits to lesser amount of expansions and shrinkages compared to the inner layer on which it is superposed.
Preferably the inner insulating layer of the covering has a dielectric strength of at least 15 kV/mm and the outer layer has a predetermined finned profile.
Typically the outer layer has a resistance to tracking at least of class 2.5 measured in accordance with Standard IEC-587, second edition, 1984.
In a preferred embodiment of the present invention, the inner layer maintains an interference of at least 10%. In addition, the inner layer is pre-expandible by at least 20%.
Preferably the inner layer maintains an interference of at least 25% and more preferably, the inner layer is pre-expandible by at least 50%.
In a further preferred embodiment of the present invention, the inner layer actuates the elastic recovery of the outer layer.
In a second aspect, the present invention relates to a tubular elastic covering for electric components characterized in having a resistance to tracking of at least class 2.5 IEC-587, and a resistance to radiation, measured with a Xenon arc lamp having a power of 6500 W at a distance of 48 cm for at least 2500 hours, such that, after a pre-expansion of at least 20%, said covering maintains an interference of at least 10%, wherein said covering comprises at least two coaxial insulating layers bonded together and made of different cross-linked polymeric material blends.
In a third aspect, the present invention relates to a termination for electric cables characterized in comprising an elastic covering in accordance with the above described coverings.
In a fourth aspect, the present invention relates to an electric component comprising a substantially cylindrical electric central element and an elastic insulating covering fitted thereon, characterized in that said covering is in a state of interference of at least 25% and in that after an exposure corresponding to a high-voltage outdoor exposure of at least 60 kV, for a time corresponding to the predetermined minimum lifetime of the electric component, said covering exhibits a surface erosion lower than 10% of the overall thickness, in which said covering comprises two layers of different elastomeric materials.
Preferably, the covering is in a state of interference of at least 25% after being kept for a predetermined period of time in a storehouse under an expansion condition of at least 50%.
In a fifth aspect, the present invention relates to a method of covering an electric element comprising the following steps:
applying a first insulating tubular layer to said electric element;
applying a second insulating tubular layer coaxial with the first one and external to said electric element;
wherein said first tubular layer has predetermined values of expandibility and elastic recovery and wherein said second tubular layer has predetermined values of resistance to tracking and resistance to solar radiation.
In addition, the steps of applying a first and a second layer to said electric element comprise the steps of:
disposing said first layer and said second layer coaxial with the first layer and external thereto, on a first removable support;
fitting said removable support with said first and second layer over said electric element;
removing said removable support causing said first and second layers to radially collapse towards said electric element.
Preferably, said second layer comprises a predetermined finned profile.
This involves the further advantage that the termination can be made in the form of a single xe2x80x9cmonolithicxe2x80x9d body, which enables testing at the factory, thereby improving the final reliability in use and simplifying mounting in the field. In addition, due to the above, said elastic coverings can be stored ready for installation.
Finally, in a sixth aspect, the present invention relates to a method of manufacturing a covering body having a predetermined profile for electric components, said method comprising the following steps:
making a first insulating layer of a cross-linkable polymeric material;
making a second insulating layer coaxial with the first layer and external thereto in a different cross-linkable polymeric material, comprising said predetermined profile;
vulcanizing at least one of said inner and outer layers.
Preferably said first layer and said second layer are manufactured and vulcanized separately and are then coaxially superposed upon each other and bonded together.
Typically, said first and second layers are bonded together by interposing an adhesive layer there between. Preferably said adhesive layer comprises a cross-linkable raw rubber layer or a glue.
In a preferred embodiment in accordance with the present invention, first of all the second outer layer is made and then vulcanized; subsequently the material used to make the first inner layer is transferred to the inside of the second vulcanized layer; afterwards the first layer too is vulcanized, so that it is bonded to the second layer.
In an alternative preferred embodiment the first layer is made by moulding, whereas the second layer is formed of two separate parts at least one of which is vulcanized before being disposed coaxially externally of the first layer.
Preferably said predetermined profile is formed on the outer surface of said second layer by an operation involving removal of material portions.